5  Baseline Reference Points

The goal of fishery management is to determine how many (numbers) or how much (weight) fish can be safely harvested from a stock. Or how many fish in a stock can die and still allow the stock to maintain itself. The rate at which fish die refers as mortality or the mortality rate.

Each year some fish die whether they are harvested or not.The rate at which fish die of natural causes is called natural mortality and the rate at which fish die from fishing is called fishing mortality. Often times the biological and exploitation reference points are used to inform management that state of the fishery resource. Some of the information needed to understand the stock of the fishery are includes;

5.1 Information about fishery

  1. The kinds of fishermen in the fishery (trawlers, longliners, gillnetters, etc.).
  2. Quantity of fish caught by each kind of fisherman over many years.
  3. Fishing effort expended by each kind of fisherman over many years.
  4. The age structure of the fish caught by each group of fishermen.
  5. The ratio males to females in the catch.
  6. How the fish are marketed (preferred size, etc.).
  7. The value of fish to the different groups of fishermen.
  8. The time and geographic area of best catches.

5.2 Information about biology

  1. The age structure of the stock.
  2. The age at first spawning.
  3. Fecundity (average number of eggs each age fish can produce).
  4. Ratio of males to females in the stock.
  5. Natural mortality (the rate at which fish die of natural causes).
  6. Fishing mortality (the rate at which fish die of being harvested).
  7. Growth rate of the fish.
  8. Spawning behavior (time and place).
  9. Habitats of recently hatched fish (larvae), of juveniles and of adults.
  10. Migratory habits.
  11. Food habits for all ages of fish in the stock.
  12. Estimate of the total number or weight of fish in the stock

To estimate the stock of fishery, we ought to have biometric information—length, weight, and maturity stage of individual fish. Since e-CAS rely on fishery dependent information, this information is not collected. Fortunately, during the SWIOFP and SWIOFISH projects, some biometric data were collected. In this chapter we discuss biological and exploitation parameters of prawn fishery. We compare these parameters for 2011 and 2020, for which data is available.

Figure 5.1: The interactive map of fisheries productive areas in Kibiti, Mafia and Kilwa Seascape

5.3 Prawn Fishery at Kibiti

A data set containing the total length and the stages of sexual maturity of 3381 prawns sampled in Mafia Channel in 2011, and 2020.

zone species cl_mm tl_mm wt_g sex maturity
2011
rumaki Metapenaeus monoceros 22.10 100 10 F I
rumaki Penaeus indicus 30.00 145 17 M I
rumaki Metapenaeus monoceros 45.00 120 11 F I
rumaki Metapenaeus monoceros 30.20 130 17 F II
2018
rumaki Penaeus indicus 28.37 154 17 F II
rumaki Metapenaeus monoceros 23.31 80 6 F I
rumaki Metapenaeus monoceros 27.78 91 13 F I
rumaki Metapenaeus monoceros 21.78 75 7 F I
2020
rumaki Penaus semisukatus 25.40 127 16 F III
rumaki Metapenaeus monoceros 23.00 110 13 F I
rumaki Fenneropenaeus indicus 36.25 100 31 F II
rumaki Metapenaeus monoceros 17.11 104 9 F I

5.4 Maturity length(L50)

Figure 5.2: The length at 50 maturity and classified adult and juvenile individual of prawn fishery in 2011 (top panels) and 2020(bottom panel)

5.5 Adult & Juvenile Ratio

5.5.1 Age

5.5.2 Selection Length